Water is a vital necessity in every-day life, whether for drinking, cooking, or industrial purposes.  It is recognized as a carrier of disease and the purification of water-supply in large cities is an important problem.  Chlorination processes are in use which render the treated water disagreeable to the taste and filtration alone is looked upon with suspicion.  The use of chemicals requires constant analysis, but it is contended that the bactericidal action of ultra-violet rays is so certain and complete that there is never any doubt as to the sterilization of the water if it is clear, or if it has been properly filtered before treating.  The system of sterilization by ultra-violet rays is the natural way, for the sun’s rays perform this function in nature.  Apparatus for sterilization of water by means of ultra-violet rays is built for public plants in capacities up to ten million gallons per day and these units may be multiplied to meet the needs of the largest cities.  Large mechanical filters are used in conjunction with these sterilizers, and thus mankind copies nature’s way, for natural supplies of pure water have been filtered through sand and have been exposed to the rays of the sun which free it from germ life.

Some sterilizers of this character are used at the place where a supply of pure water is desired or at a point where water is bottled for use in various parts of a factory, hospital, store, or office building.  These were used in some American hospitals during the recent war, where they supplied sterilized water for drinking and for the antiseptic bathing of wounds.  In warfare the water supply is exceedingly important.  For example, the Japanese in their campaign in Manchuria boiled the water to be used for drinking purposes.  The mortality of armies in many previous wars was often much greater from preventable diseases than from bullets, but the Japanese in their war with Russia reversed the mortality statistics.  Of a total mortality of 81,000 more than 60,000 died of casualties in battle.

The sterilization of water for swimming-pools is coming into vogue.  Heretofore it was the common practice to circulate the water through a filter, in order to remove the impurities imparted to it by the bathers and to return it to the pool.  It is insisted by the adherents of sterilization that filtration of this sort is likely to leave harmful bacteria in the water.  Sterilizers in which ultra-violet rays are the active rays are now in use for this purpose, being connected beyond the outflow from the filter.  The effectiveness of the apparatus has been established by the usual method of counting the bacteria.  Near the outlet of the ordinary filter a count revealed many thousand bacteria per cubic inch of water and among these there were bacteria of intestinal origin.  Then a sterilizer was installed in which the effective elements were two quartz mercury-lamps which consumed 2.2 amperes each at 220 volts.  A count of bacteria